Technique creates “islands” to isolate optical energy in skin

In 2005, Americans
spent more than $300 million on almost 800,000 laser hair removal treatments, according
to the American Society of Plastic Surgeons. Lasers also can be used to alter skin
pigmentation or to remove unwanted wrinkles, scars and tattoos.

However, skin scatters light, and optical energy
must penetrate below its top layer for effective dermatological treatment. It also
must be focused on the treatment area to avoid damaging surrounding tissue. Optical
clearing agents can cause the skin to have a more uniform refractive index, thus
reducing light scatter, but they must permeate the skin to be effective. It is possible
to use an erbium laser to remove the top layer of skin before applying optical clearing
agents, to help them reach the underlying layers, but this results in an open wound.

Therefore, Ilya V. Yaroslavsky and
his colleagues at Palomar Medical Technologies Inc. in Burlington, Mass., and at
Saratov State University in Russia tested the effectiveness of another technique
to enhance the ability of optical agents to penetrate the skin without significantly
compromising their protective function.

They applied a pattern of small, light-absorbing
carbon dots — aligned in rows and columns — to the skin and illuminated
its surface with intense pulsed light from a flashlamp. The procedure formed a “lattice
of islands of thermal damage” that served as routes for the agent to penetrate
without disturbing the tissue between the islands.

The scientists placed carbon
dots in a lattice formation on the skin of healthy human volunteers and on human
and pig ex vivo skin samples, followed by topical application of an optical clearing
agent and illumination with intense light pulses. As a result, light transmittance
through the skin increased by an average of three to 10 times.
The researchers used fresh rat, farm
pig and Yucatan pig skin samples (in order of increasing similarity to human skin
and decreasing cost and availability), followed by tests on healthy human volunteers.
Yaroslavsky said that they used various optical clearing agents, including glycerol,
propylene glycol and glucose, because there is a lack of agreement in the literature
as to which one works best. The carbon dots were approximately 75 to 120 μm
in diameter and were spaced about 450 to 500 μm apart. Those measurements were
empirically determined to result in the best compromise between skin penetration
and safety. The total area of the lattice matched that of the flashlamp window.
They employed two flashlamp systems, the EsteLux and MediLux from Palomar.

On the volunteers’ skin, the
researchers applied a blue dye. After using the flashlamp with a 20-ms pulsewidth
and a fluence of 14 J/cm2, they observed that the dye entered the skin, demonstrating
increased permeability. On the farm pig sample, they applied Lux lotion from Palomar
and a 40 percent glucose solution. After operating the flashlamp with a fluence
of 36 J/cm2 and a 20-ms pulsewidth, they saw maximal optical clearing 60 minutes
after topical application of the optical clearing agent, which was assessed by the
visual appearance of blue wires in the specimen. For 20 J/cm2 and a 10-ms pulsewidth,
they saw less optical clearing.

They also placed sheets of paper with
the printed word “clearing” or the letter “O” under rat
skin, and the writing appeared after treatment. Coating the rat skin with glycerol
and using the flashlamp with the same pulsewidth and fluence resulted in maximal
optical clearing after 60 minutes, but using 40 percent glucose had a less noticeable
effect. Overall, glycerol was the most effective optical clearing agent.

The researchers then employed a BioSpec
holographic-grating spectrophotometer to measure the intensity of light that passed
through the skin. For Yucatan pig, farm pig and human skin samples, the relative
transmittance (the ratio of intensity after treatment to that before treatment)
increased by an average of three to 10 times when the lattice of islands of damage
was used.

The scientists said that these results
demonstrate the effectiveness of the technique, but they believe that further
research is necessary for its optimization.